1. Field of the Invention
This invention relates generally to the fabrication of giant magnetoresistive (GMR) magnetic field sensors of a “current-perpendicular-to-the-plane” (CPP) configuration and more particularly to such a sensor that includes a novel current channeling layer (CCL) that effectively lowers the high parasitic resistance of an antiferromagnetic pinning layer.
2. Description of the Related Art
Magnetic read sensors that utilize the giant magnetoresistive (GMR) effect for their operation are generally of the “current-in-the-plane” (CIP) configuration, wherein current is fed into the structure by leads that are laterally disposed to either side of an active sensor region and the current moves through the structure essentially within the planes of its magnetic and other conducting layers. Since the operation of GMR sensors depends on the detection of resistance variations in their active magnetic layers caused by changes in the relative directions of their magnetic moments, it is important that a substantial portion of the current passes through those layers so that their resistance variations can have a maximally detectable effect. Thus, one problem associated with the CIP formation is that the signal it produces becomes scaled down together with the reduction in its trackwidth. Since this is not a problem with the CPP configuration, the CPP configuration holds the promise of being effective in reading magnetically recorded media having recording densities exceeding 100 Gbit/in2. In addition, the CPP configuration has the benefit of a very low resistance, which also makes it more suitable for very small sensor sizes.
The CPP configuration is not without its problems, however. A significant problem inherent in the CPP configuration is the large voltage drop across magnetically inactive high resistance layers, which tends to mask the voltage variations produced by the active layers. The GMR resistance ratio, ΔR/R, is typically on the very low order of 1% for the CPP design, because the ΔR is provided by variations of the low resistance, magnetically active layers, whereas R includes the high resistance of inactive layers. It is worth noting that the high value of R also increases Joule heating in the sensor and, therefore, limits the allowable magnitude of the sensing current.
GMR stack designs favor the use of magnetically pinned layers that are pinned by antiferromagnetic (AFM) pinning layers. Antiferromagnetic materials used in such pinning layers tend to be formed of high-resistance materials and it is these layers that provide a parasitic resistance, Rpa, that is included in R and lowers the sensitivity, ΔR/R, of the CPP sensor.
One approach to alleviating this problem is to discover and use low-resistance AFM materials. This would necessitate a difficult materials search. An alternative approach is to lower the effective parasitic resistance of the AFM layer by changing the sensor geometry and adding a novel current channeling layer. That is the approach taken by the present invention.
Prior art has offered no similar method for improving the sensitivity of the CPP design. Yuan et al. (U.S. Pat. No. 5,883,763) disclose a CPP sensor wherein a GMR layer is positioned between upper and lower permanent magnet biasing layers and the whole structure is then positioned between an upper and lower shield. Conducting layers are formed on the upper and lower surfaces of the biasing layers to separate them from the shields and the GMR layer. Although this approach offers the benefits of improved magnetic biasing, it does not deal with the problem of parasitic resistance.
Yuan et al. (U.S. Pat. No. 5,731,937) discloses a CPP sensor configuration having sensing element dimensions in a particular ratio to current lead dimensions so that the efficiency of the element is thereby increased.
Yuan et al. (U.S. Pat. No. 5,739,987) discloses a GMR transducer assembly operating in a CPP mode and being magnetically biased by a multilayer biasing structure comprising alternating layers of ferromagnetic and antiferromagnetic material.
Mao et al. (U.S. Pat. No. 6,411,478) discloses a spin tunnel junction formed between a bottom shield and a shared pole and separated from each of them by dielectric gaps wherein the spin tunnel junction comprises a magnetically free layer separated symmetrically from two longitudinally disposed pinned layers by edge junctions. Conducting leads contacting the two pinned layers on opposite lateral edges of the configuration feed current through the spin tunnel junction is a CPP configuration.
Sin et al. (U.S. Pat. No. 6,353,318) provides a method for forming a CPP sensor having hard bias layers positioned so as not to allow shorting between the current carrying leads.
The prior art described above does not address the problem of the parasitic resistance of an AFM pinning layer and its adverse affect on sensor sensitivity. In particular, the prior art does not discuss or disclose a method of forming a CPP GMR sensor in which the AFM pinning layer is configured in a novel way and separated from the remainder of the sensor stack by a current channeling layer.